Wide spread use of home video recording and playback equipment has increased the demand for prerecorded cassettes, especially half-inch video cassettes. Fast production of such cassettes is difficult, because video signals contain a large amount of information which must be reproduced with a high degree of accuracy to maintain adequate image and color standards.
At present, the preferred video recording method used by this industry is to use upward of 1,000 recording machines, all operating simultaneously, all receiving a realtime signal from one master playback machine to produce multiple copies. The logistics of providing, loading, unloading, operating and maintaining such multitude of relatively complex equipment makes reproduction a very expensive process.
Another approach to magnetic recording duplication is non-realtime duplication known as magnetic replication. In this type of duplication, the signal pattern from a master tape is replicated directly on the receiving tape at very high speed. The magnetic pattern in this process is printed onto a receiving tape in much the same way as a visible image is printed through a high speed press onto a sheet of paper. Copy speeds in excess of 100 times realtime duplication have been reported.
Tape replication is obtained through either of two well known processes. One comprises bringing the surface of a master tape having a very high degree of coercivity into intimate contact with the surface of a conventional low coercivity magnetic tape in the presence of a magnetic field and removing the external field while maintaining contact. This is known as anhysteretic tape replication and is described in an article by Y. Odagiri and T. Sato entitled "High Speed Video Tape Duplication" which appeared in the August 1984 IEEE transactions on Consumer Electronics.
The second method also involves bringing the two tape surfaces into intimate contact, and then momentarily raising the temperature of the copy tape to above the magnetic material's Curie temperature; while the two surfaces are still in contact, the tape is cooled below its Curie temperature and then the tapes are separated. This method, is known as thermal magnetic replication: It is, of course, essential that the copy tape have a low Curie temperature when thermal magnetic replication is used to prevent heat damage to the polyester supporting base of the tape. The master tape on the other hand should have a high Curie temperature.
Thermal magnetic duplication is also well known in the art and described in various publications, such as U.S. Pat. No. 4,213,159 issued to King, as well as this assignee's copending application U.S. Ser. No. 774,885 which describes a thermal replication system and associated apparatus in which a laser beam is used to momentarily heat a copy tape having a Curie temperature of below 130.degree. C. in contact with a master tape having a Curie temperature above 500.degree. C. while both tapes are under pressure in a nip formed between a glass roller and an elastomeric pressure roller.
Experience with both real and non realtime tape duplication indicates that close contact is required between tape to tape or tape to magnetic head surfaces in order to assure maximum signal transfer, especially in the reproduction of short wavelengths. Thus, the trend in the art is toward the development of smooth surface tapes.
In order for non realtime duplicating system to be commercially successful and to compete with the realtime equipment, it is desirable to copy the magnetic pattern from the master tape onto the copy tape at very high speed, since in this process only one tape is produced at a time. To increase replication speed, it has been proposed to use a looped master tape together with a large supply of blank copy tape for uninterrupted running. After replication, the copy tape is cut to the appropriate length for each copy.
As the replication speeds are increased, it becomes progressively more difficult to maintain contact and registration of the master and copy tape with the required degree of accuracy to produce an acceptable signal transfer which will generate a high quality video signal for eventual use in home video cassette recorder and viewing equipment. In the aforementioned article by Odagiri et al., this problem has been recognized and a number of solutions have been proposed, such as the use of air pressure to maintain contact between the tapes, air bearings to provide smooth tape running, an auxiliary "driving tape" to drive the tapes through the printing station, and smooth surface tapes to assure intimate contact between the two tapes by reducing the spacing between the master and copy tape surfaces.
The problems recognized by Odagiri and present in the anhysteretic magnetic replication method are also present in the thermal magnetic replication system, since they are inherent to high speed reproduction. Briefly stated, the challenge consists of bringing two tape surfaces momentarily into intimate contact as they are driven at very high speed through a printing station, without any relative slippage. If the tapes are not in contact because of air entrapment between the surface, they either completely float apart or develop pockets of entrapped air, and the amplitude, phase or registration of the transferred signal will be diminished or altered. If the tapes, because of floating or other drive irregularities, also slip relative to each other, the replication will no longer be true, and the result will again be a loss of signal amplitude on playback. This may exhibit a periodicity, in which case it is referred to as "breathing" or may appear as a random effect.
In an effort to alleviate these problems, a high pressure nip roller printing station has been used. However, registration and contact problems resulting in uneven playback signal reappeared in the prior art equipment employing nip rollers for thermomagnetic duplication at speeds of about 60 to 90 inches per second. At high speed, "breathing", again degrades the signal limiting the speed of duplication. It appears that "breathing" is the result of mechanical tape drive problems, such as edge alignment of the copy and master tape, or floating, which become more evident as higher tape duplication speeds are attempted. Higher and higher pressure in the nip tends to alleviate "breathing". However, as the relative pressure increase, the mechanical problems of registration, drive and concentricity of the rollers become accentuated, introducing other sources of signal degradation. There is thus still need for a method to produce high quality duplicate tapes at speeds in excess of 100 inches per second (2.5 meters/second) by alleviating the signal degradation.